Field of the Invention
The present invention relates to a new lettuce (Lactuca sativa) variety which exhibits resistance against downy mildew (Bremia lactucae).
Description of Related Art
All cultivated forms of lettuce belong to the highly polymorphic species Lactuca sativa that is grown for its edible head and leaves. As a crop, lettuces are grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of diseases such as downy mildew, sclerotinia rot, botrytis, powdery mildew, anthracnose, bottom rot, corky root rot, lettuce mosaic virus, big vein, beet western yellows and aster yellows. These diseases result in millions of dollars of lost lettuce crop throughout the world every year.
Of the various diseases that affect lettuce cultivars, downy mildew (Bremia lactucae) is the most highly destructive of lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae Regal, which can be one of the following strains: NL1, NL2, NL4, NL5, NL6, NL7, NL10, NL12, NL13, NL14, NL15, NL16, B1:17, B1:21 and B1:23 (Van Ettekoven, K. et al., “Identification and denomination of ‘new’ races of Bremia lactucae.” In: Lebeda, A. and Kristkova, E (eds.), Eucarpia Leafy Vegetables, 1999, Palacky University, Olomouc, Czech Republic, pp. 171-175).
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Although several known lettuce cultivars exhibit resistance against downy mildew, irrespective of lettuce type, all the lettuce cultivars affected produce a limited number of leaves that generally are of unequal size and diminished quality with respect to color and shape. This is a distinct disadvantage for processing purposes because leaves either need to be sorted based on size or they need to be cut to a smaller, more uniform size. The first option requires additional labor, with not all sizes usable. The second option has the disadvantage that it creates many cut surfaces which then are subject to wound-induced browning, resulting in a greatly reduced shelf-life.
A need exists, therefore, for an improved lettuce variety which exhibits resistance to downy mildew and exhibits abundant leaf growth and vibrant color.